1. Field of the Invention
The present invention relates to radio frequency identification (RFID) devices, and more particularly to a system and method for setting an oscillator in an RFID transponder.
2. Description of Related Art
Radio Frequency Identification (RFID) base stations, which operate in conjunction with RFID transponders (tags), are used in a variety of applications (e.g., inventory-control, security, etc.). Typically, an item including an RFID tag (e.g., a container with an RFID tag inside) is brought into a “read zone” established by the base station. The base station transmits an interrogating RF signal that is modulated, in part, by the receiving tag. That is, in reading the tag, the base station generates and transmits a continuous wave electromagnetic disturbance at a particular carrier frequency. This disturbance is then modulated by the receiving tag in order to impart information (e.g., information stored within the tag) onto the signal. The modulated RF signal is then reflected back to the base station (i.e., backscattered) where the imparted information is extracted.
The frequency at which information is modulated onto the received RF signal is determined by the RFID tag, or more particularly, oscillation circuitry located therein. Traditionally, RFID tags include a logic circuit, a volatile memory device and a variable oscillator that can be programmed to oscillate at a particular frequency. The logic circuit is adapted to identify (i) the frequency at which the received RF signal is transmitted, known as the carrier frequency, and (ii) the frequency at which information has been modulated onto the received RF signal, known as the modulation frequency. The modulation frequency is then stored in the volatile memory and used by the variable oscillator, to generate a similar (or same) frequency. The generated frequency is then used by the logic circuit to modulate information (e.g., information stored in non-volatile memory, etc.) onto the received RF signal.
This allows the RFID tag to transmit information at the same frequency as it was received (or at a known relationship thereto). It also allows the RFID base station to receive information modulated at the same frequency as it was sent (or at a known relationship thereto). In other words, for example, if the RFID base station transmits an RF signal that includes information modulated at a particular frequency (e.g., 120 kHz), then the reflection of that signal should also include information modulated at the same (or substantially the same) frequency (e.g., 120 kHz).
One of the drawbacks associated with such a system is that analog circuits, such as variable oscillators, generally require a large amount of real estate and consume a large amount of power. This is especially true when analog circuits are compared to digital circuits.
Another drawback associated with such a system is that the variable oscillator needs to be calibrated every time the RFID tag enters a new “read zone.” This is because the deduced modulation frequency (as identified by the logic circuit) is only stored in volatile memory, which is erased each time power to the memory device is lost or turned off. This is important because in passive RFID tags, where power is derived from received RF signals, the volatile memory is not being powered when the tag is between “read zones” and not receiving RF signals. It is also important in active RFID tags, which include an on-board power supply, because the tag is sometimes placed into a “sleep” state when it is between “read zones,” which may result in power being removed (at least temporarily) from the memory device(s). If the deduced modulation frequency is erased, then the RFID tag is required to recalibrate its oscillation circuit every time it enters a “read zone,” which wastes time that otherwise could be used to communicate information.
Thus, it would be advantageous to provide an oscillation circuit in an RFID tag that does not suffer from at least one of these drawbacks.